Noise in Industry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 618

three primary instruments (sound level meters, dosimeters, and real-time spectrum analyzers) and their
data output will suffice. In instances where noise is highly impulsive in nature or selection and develop-
ment of situation-specific engineering noise control solutions is anticipated, more specialized instru-
ments may be necessary.
Because sound is propagated as pressure waves that vary over space and in time, a complete quanti-
fication would require simultaneous measurements over the continuous time periods (representing com-
plete operator exposure durations) at all points of an occupational sound field to exhaustively document
the noise level in the space. Clearly, this is typically cost- and time-prohibitive, so one must resort to
sampling strategies for establishing the observation points and intervals. The hearing conservationist
must also decide whether detailed, discrete time histories are needed (such as with a noise-logging dosi-
meter, discussed later), if averaging over time and space with long data records is required (with an aver-
aging integrating dosimeter), whether discrete samples taken with a short-duration moving time average

(with a basic sound level meter) will suffice, or if frequency-band-specific SPLs are needed for selecting
noise abatement materials (with a spectrum analyzer). Following is a brief discussion of the three primary
types of sound measurement instruments and the noise descriptors that can be obtained therefrom.

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31.4.1.1 Sound Level Meter
Most sound measurement instruments derive from the basic sound level meter (SLM), a device for which
four grades and associated performance tolerances that become more stringent as the grade number
increases are described in ANSI S1.4-1983 (R2001). 17 Type 0 instruments have the most stringent toler-
ances and are for laboratory use only. Other grades include Type 1, intended for precision measurement
in the field or laboratory, Type 2, intended for general field use, especially where frequencies above
10,000 Hz are not prevalent, and Type S, a special purpose meter that may perform at grades 1 to 3,
but may not include all of the operational functions of the grade. A grade of Type 2 or better is
needed for occupational exposure measurements.

31.4.1.1.1 Components of a Sound Level Meter
A block diagram of the functional components of a generic SLM is given in Figure 31.4. At the top, a
microphone

preamplifier senses the pressure changes caused by an airborne sound wave and converts
the pressure signal into a voltage signal. Because the pressure fluctuations of a sound wave are small
in magnitude, the corresponding voltage signal must be preamplified and then input to an amplifier
that boosts the signal before it is processed further. The passband, or range of frequencies that are
passed through and processed, of a high-quality SLM contains frequencies from about 10 to
20,000 Hz, but depending on the frequency weighting used, not all frequencies are treated the same.
A selectable frequency weighting network, or filter, is then applied to the signal. These networks most
commonly include the A-, B-, and C-weighting functions shown in the bottom panel of Figure 31.2.
For OSHA noise monitoring measurements, the A-scale, which de-emphasizes the low frequencies
and to a smaller extent the high frequencies, is used. In addition to the common A scale (which approxi-
mates the 40-phon level of hearing) and C scale (100-phon level), other scales, including dB(linear), may
be included in the meter.
Next (not shown), the signal is squared to reflect the fact that the sound pressure level in decibels is a
function of the square of the sound pressure. The signal is then applied to an exponential averaging
network, which defines the meter's dynamic response characteristics. In effect, this response creates a
moving-window, short-time average display of the sound waveform. The two most common settings
are defined as FAST, which has a time constant of 0.125 sec, and SLOW, which has a time constant of
1.0 sec. These time constants were established decades ago to give analog needle indicators a rather slug-
gish response so that they could be read by the human eye even when highly fluctuating sound pressures
were measured. Under the FAST or SLOWdynamics, the meter indicator rises exponentially toward the
decibel value of an applied constant SPL. In theory, when driven by an exponential process, the indicator
would reach the actual value at infinite time; however, the time constant defines the time period within
which the indicator reaches 63% of the maximum value in response to a constant input. For OSHA

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